Nazeing also had this shape in their 1930s catalogue as Posy Bowl in two styles of finish and several sizes up to 14 inches, again it would not be distinctly different to yours. And their is at least one other type regularly seen that is generally thought to be 30s-50s and always gets described as Monart or Nazeing but was neither.

I wonder which company first derived the mushroom shape? Monart, perhaps, or was it much earlier? The shapes can be seen from pottery companies as well, of course, so it might have been copied from here.